Presently available communications systems primarily use circuit-based switching transmission systems. However, asynchronous transfer mode (ATM) switch technology has recently become available, giving rise to new choices. ATM is used within telecommunications switching networks and may carry a wide variety of traffic types from different applications such as voice data, image data, fax data, modem data, video data, computer data, multimedia data, and program code data. Furthermore, the use of code division multiple access (CDMA) for wireless communications systems is becoming a method of choice for data and voice transport, due to its inherent variable rate traffic channel frame organization. The influence of variable-length packet transport inherent to CDMA, and fixed-length cell-switching inherent to ATM, requires efficient conversion techniques at the interface between the wireless transport and the switching network. In the conversion of data streams into ATM cells, the presently available systems focus on transporting CDMA speech packets via variable rate packets by utilizing off-the-shelf segmentation and reassembly (SAR) devices, or data optimized conversion devices, yielding a more complex and costly implementation.
Existing commercial off-the-shelf SAR devices are available to perform constant-bit-rate (CBR) and/or variable-bit-rate (VBR) traffic for ATM cell conversion. SAR involves chopping packets into cells and reassembling them for multiple-receive cells to form a single large packet. More information on off-the-shelf SAR devices may be found in IgT AAL5 SAR Processor, WAC-020-A; Data Sheet, Motorola SPS ATM AAL SAR Controller With Framer, MC92516; Data Sheet, TI SAR Device 0 with PCI Host Interface, TNETA1562; Data Sheet, TI SAR Device with Integrated 64-bit PCI Host Interface; and, TNETA1570: Data Sheet. However, these devices were developed for desktop applications and are primarily intended to handle data (i.e. non-voice) information. Furthermore, as a result of the intended application, most devices must operate within a PCI-bus environment, relying upon either an integrated or external controller and host-memory. Also, in order to expand the range of applications in which these devices may operate, embedded firmware is typically used to perform a specific ATM adaptation layer (AAL) conversion. Therefore, the AAL may be changed by downloading and enabling new firmware. However, the dependence upon firmware to process-differing AALs, as well as upon PCI architectures for interacting with computer-based peripheral devices, along with the inability to perform per-VC mixed mode AALs (e.g., AAL 1 or AAL 5 simultaneous on various VC) limits the usefulness of such devices in voice applications More information on PCI architecture may be found in PCI Local Bus Specification, Rev 2.0, PCI Special Interest Group, Hillsboro, Oreg., or PCI System Design Guide, Rev 1.0, PCI Special Interest Group, Hillsboro, Oreg.
In particular, PCS systems employing the CDMA air-interface utilize variable-rate speech codecs to efficiently utilize the frequency spectrum and to minimize cross-interference among other simultaneous users. Two methods (synchronous backhaul and asynchronous backhaul) are commonly known in the art for back hauling downconverted and digitized data from a base termination site (BTS) to a transcoder for conversion into a format compatible with the public switch telephone network (PSTN). Since the BTSs are located at separate locations from a central base site controller (CBSC), separate point-to-point span line interfaces are typically provided between the BTS and the CBSC for backhaul. Backhaul relates to the mapping of information frames onto physical media (e.g., electrical or optical) for transport between the CBSC and the BTSs. Network operators seek to minimize backhaul costs associated with the physical access plant containing the physical connections in a communications system. Different techniques are utilized to achieve this operating efficiency, such as attaining higher utilization (i.e., transport of more calls) on the same fabric.
A synchronous backhaul approach may be utilized, whereby the variable-rate voice packet is segmented into one or two bit subslots and sent via a 16 kbps synchronous channel via either a T1 or E1 span line. More information on this approach may be found in TIA TR46 Ad-Hoc Group, "Voice Frame Format" by Karl Lewis of Motorola, Aug. 28, 1995. The CDMA air-frame is converted into a synchronous, constant-bit size packet, consisting of 320 bits that are sent at 2 bits per 125 microsecond frame over 160 frames. The synchronous backhaul approach utilizes a 320-bit fixed-length packet that contains the CDMA voice data, synchronization information, and other overhead. By contrast, the asynchronous backhaul approach utilizes variable-sized packets typically ranging from 6 to 38 bytes plus overhead for CDMA. The appropriate AAL technique for carrying variable rate data, such as voice/data, is AAL-CU (also referred to as AAL2).
A second backhaul approach may be utilized whereby the variable-sized packet is encapsulated in an asynchronous packet (frame relay, FUNI (Frame User Network Interface), etc.) and sent via a frame format via either a TI or E1 span line. The advantage of the asynchronous technique over the synchronous backhaul technique is that the available bandwidth is used more efficiently, since only relevant information is sent per packet transfer in the asynchronous data stream--versus a fixed 320 bits for the smallest packet size in the synchronous data stream. The asynchronous interface partially utilizes a single AAL2-formatted ATM cell payload (occupying from 8 to 24-bytes plus overhead). Thus, more subscribers/calls may be transported over the same span line than may be supported by the synchronous approach.
However, the advantage of the synchronous backhaul technique over the asynchronous backhaul technique is that processing is simplified. Furthermore, the synchronous backhaul technique is compatible with the embedded infrastructure of circuit switching network topology. As ATM cell-switching experiences rapid growth, an efficient solution must be provided to realize the bandwidth efficiencies of newer asynchronous/packet backhaul while ensuring backward compatibility with presently used synchronous backhaul in personal communication system (PCS)/CDMA communications system. In either case (synchronous or asynchronous), the data is optimally transferred within the CBSC utilizing a cell switching system. Therefore, the packet or frame from the span line must be converted into an ATM cell utilizing the appropriate ATM adaptation layer (AAL) technique, AAL 5
The AAL5 method may support packet sizes ranging from 0 to 64 k octets by segmenting each packet into smaller sized groupings for encapsulation within an ATM cell payload. The final cell utilizes 8 octets of the 48-octet payload for AAL 5-specific information. However, observation of the data format present within a CDMA air-frame for 1/8; 1/4; 1/2; and full rate speech shows that the entire air-frame may be encapsulated within a single AAL 5-format.
Currently, the most prevalent method of transporting data between the BTS and the CBSC is via the synchronous backhaul approach. However, cell-based (asynchronous) backhaul approaches promise better utilization of the backhaul transmission fabric, since it utilizes only the amount of bandwidth required rather than a fixed amount of bandwidth, regardless of the amount occupied (as required by the synchronous backhaul approach). The introduction of such asynchronous-based backhaul is therefore dependent upon an efficient method of switching packets within a Base Station Subsystem (BSS).
It is therefore desirable to provide a BSS that is independent of a Mobile Switching Center (MSC). Further, a system that provided a switching system that utilized cell-based switching, while maintaining compatibility with both cell and circuit-based transmission formats, would be an improvement over the prior art.